Evaluation of Aerosol Optical Properties and Black Carbon at the Mt. Bachelor Observatory Mountain-Top Site

The location of the Mount Bachelor Observatory  (MBO; 2764 meters asl), is an ideal site for the frequent sampling of the free troposphere and aged biomass burning plumes from regional and global sources. As one of the largest sources of black carbon (BC) globally, it is important that we have a quantitative assessment of the aerosol radiative forcing contribution from biomass burning events. Forest Fire emissions are likely to increase because of climate change, particularly in the boreal forests of North America and Russia and also the western U.S. thereby adding further urgency to this measurement and modeling effort.

A Single-Particle Soot Photometer (SP2; Droplet Measurement Technologies) obtained through the DOE-ARM (Atmospheric Radiation Measurement) instrument loan program was deployed at MBO during the summer of 2016 along with a 7-wavelength Aethalometer (AE33; Magee Scientific) borrowed from the NOAA ESRL Global Monitoring Division. These instruments complemented the core measurement suite at MBO that includes CO, CO₂, O₃, aerosol scattering (σ_scat) and absorption (σ_ab), PM mass, and aerosol size distribution.

The SP2 provides a particle-resolved measurement of refractory black carbon (rBC) mass concentration and size distribution. The ΔrBC/ΔCO enhancement ratios and rBC mass fraction (ΔrBC/ΔPM) will be calculated for biomass burning events and their dependence on plume age, plume origin, and fire characteristics will be evaluated. The absorption enhancement (Δσ_ab/ΔCO) and Ångström absorption exponent (AAE) of biomass events will be compared with rBC mass and size, rBC coating thickness to determine the influence of brown carbon (BrC) as a function of plume age and origin. In addition we will compare the size distribution of rBC-containing particles in biomass burning measured by the SP2 with total particle size distributions measured by the Scanning Mobility Particle Sizer (SMPS).

An evaluation of rBC mass measured by the SP2 compared to BC estimated by the Aethalometer will access the robustness of the correction schemes for this filter-based technique. The outcome of these analyses will be discussed with the goal of better understanding the role of BC in aged biomass burning plumes and its influence on the radiative forcing by these events.